4,025 research outputs found
Photon-pair generation by non-instantaneous spontaneous four-wave mixing
We present a general model, based on a Hamiltonian approach, for the joint
quantum state of photon pairs generated through pulsed spontaneous four-wave
mixing, including nonlinear phase-modulation and a finite material response
time. For the case of a silica fiber, it is found that the pair-production rate
depends weakly on the waveguide temperature, due to higher-order Raman
scattering events, and more strongly on pump-pair frequency detuning. From the
analytical model, a numerical scheme is derived, based on the well-known
split-step method. This scheme allows computation of joint states where
nontrivial effects are included, such as group-velocity dispersion and Raman
scattering. In this work, the numerical model is used to study the impact of
the non-instantaneous response on the pre-filtering purity of heralded single
photons. We find that for pump pulses shorter than 1 ps, a significant
detuning-dependent change in quantum-mechanical purity may be observed in
silica
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Long-term Framework for Electricity Distribution Access Charges
In order to achieve overall economic efficiency, incentive regulation of electricity distribution utilities must address two important and inter-related issues. First, the utilities’ allowed revenues need to be set at correct levels. Second, the access charging mechanism by which the utilities recover the allowed revenues must give the correct economic signals to generation and load connected to the network. This paper is concerned with the latter aspect of regulation. The paper discusses the main economic principles that should form the basis on which a distribution access charging model is developed. The charging model should have a number of attributes: be calibrated to each existing network; contain an asset register; be able to determine assets needed to meet new demand; find least-cost system expansion; compute network losses and handle ancillary services; estimate incremental operating and maintenance costs; be available to users; and be simple enough for external users to understand
Hybridization at superconductor-semiconductor interfaces
Hybrid superconductor-semiconductor devices are currently one of the most
promising platforms for realizing Majorana zero modes. Their topological
properties are controlled by the band alignment of the two materials, as well
as the electrostatic environment, which are currently not well understood.
Here, we pursue to fill in this gap and address the role of band bending and
superconductor-semiconductor hybridization in such devices by analyzing a gated
single Al-InAs interface using a self-consistent Schrodinger-Poisson approach.
Our numerical analysis shows that the band bending leads to an interface
quantum well, which localizes the charge in the system near the
superconductor-semiconductor interface. We investigate the hybrid band
structure and analyze its response to varying the gate voltage and thickness of
the Al layer. This is done by studying the hybridization degrees of the
individual subbands, which determine the induced pairing and effective
-factors. The numerical results are backed by approximate analytical
expressions which further clarify key aspects of the band structure. We find
that one can obtain states with strong superconductor-semiconductor
hybridization at the Fermi energy, but this requires a fine balance of
parameters, with the most important constraint being on the width of the Al
layer. In fact, in the regime of interest, we find an almost periodic
dependence of the hybridization degree on the Al width, with a period roughly
equal to the thickness of an Al monolayer. This implies that disorder and shape
irregularities, present in realistic devices, may play an important role for
averaging out this sensitivity and, thus, may be necessary for stabilizing the
topological phase.Comment: 10 Figures. 16 pages. Published versio
Engineering spectrally unentangled photon pairs from nonlinear microring resonators through pump manipulation
The future of integrated quantum photonics relies heavily on the ability to
engineer refined methods for preparing the quantum states needed to implement
various quantum protocols. An important example of such states are
quantum-correlated photon pairs, which can be efficiently generated using
spontaneous nonlinear processes in integrated microring-resonator structures.
In this work, we propose a method for generating spectrally unentangled photon
pairs from a standard microring resonator. The method utilizes interference
between a primary and a delayed secondary pump pulse to effectively increase
the pump spectral width inside the cavity. This enables on-chip generation of
heralded single photons with state purities in excess of 99 % without spectral
filtering.Comment: 5 pages, 5 figure
Weak Coulomb blockade effect in quantum dots
We develop the general non-equilibrium theory of transport through a quantum
dot, including Coulomb Blockade effects via a 1/N expansion, where N is the
number of scattering channels. At lowest order we recover the Landauer formula
for the current plus a self-consistent equation for the dot potential. We
obtain the leading corrections and compare with earlier approaches. Finally, we
show that to leading and next leading order in 1/N there is no interaction
correction to the weak localization, in contrast to previous theories, but
consistent with experiments by Huibers et al. [Phys. Rev. Lett. 81, 1917
(1998)], where N=4.Comment: 4 pages, 2 figures. Published versio
Spectrally pure heralded single photons by spontaneous four-wave mixing in a fiber: reducing impact of dispersion fluctuations
We model the spectral quantum-mechanical purity of heralded single photons
from a photon-pair source based on nondegenerate spontaneous four-wave mixing
taking the impact of distributed dispersion fluctuations into account. The
considered photon-pair-generation scheme utilizes pump-pulse walk-off to
produce pure heralded photons and phase matching is achieved through the
dispersion properties of distinct spatial modes in a few-mode silica step-index
fiber. We show that fiber-core-radius fluctuations in general severely impact
the single-photon purity. Furthermore, by optimizing the fiber design we show
that generation of single photons with very high spectral purity is feasible
even in the presence of large core-radius fluctuations. At the same time,
contamination from spontaneous Raman scattering is greatly mitigated by
separating the single-photon frequency by more than 32 THz from the pump
frequency
Unraveling the acoustic electron-phonon interaction in graphene
Using a first-principles approach we calculate the acoustic electron-phonon
couplings in graphene for the transverse (TA) and longitudinal (LA) acoustic
phonons. Analytic forms of the coupling matrix elements valid in the
long-wavelength limit are found to give an almost quantitative description of
the first-principles based matrix elements even at shorter wavelengths. Using
the analytic forms of the coupling matrix elements, we study the acoustic
phonon-limited carrier mobility for temperatures 0-200 K and high carrier
densities of 10^{12}-10^{13} cm^{-2}. We find that the intrinsic effective
acoustic deformation potential of graphene is \Xi_eff = 6.8 eV and that the
temperature dependence of the mobility \mu ~ T^{-\alpha} increases beyond an
\alpha = 4 dependence even in the absence of screening when the full coupling
matrix elements are considered. The large disagreement between our calculated
deformation potential and those extracted from experimental measurements (18-29
eV) indicates that additional or modified acoustic phonon-scattering mechanisms
are at play in experimental situations.Comment: 7 pages, 3 figure
Gravitino Dark Matter in the CMSSM With Improved Constraints from BBN
In the framework of the Constrained MSSM we re--examine the gravitino as the
lightest superpartner and a candidate for cold dark matter in the Universe.
Unlike in other recent studies, we include both a thermal contribution to its
relic population from scatterings in the plasma and a non--thermal one from
neutralino or stau decays after freeze--out. Relative to a previous analysis
[1] we update, extend and considerably improve our treatment of constraints
from observed light element abundances on additional energy released during BBN
in association with late gravitino production. Assuming the gravitino mass in
the GeV to TeV range, and for natural ranges of other supersymmetric
parameters, the neutralino region is excluded, while for smaller values of the
gravitino mass it becomes allowed again. The gravitino relic abundance is
consistent with observational constraints on cold dark matter from BBN and CMB
in some well defined domains of the stau region but, in most cases, only due to
a dominant contribution of the thermal population. This implies, depending on
the gravitino mass, a large enough reheating temperature. If \mgravitino>1
GeV then GeV, if allowed by BBN and other constraints but, for light
gravitinos, if \mgravitino>100 keV then GeV. On the other
hand, constraints mostly from BBN imply an upper bound T_R \lsim {a few}x
10^8\times10^9 GeV which appears inconsistent with thermal leptogenesis.
Finally, most of the preferred stau region corresponds to the physical vacuum
being a false vacuum. The scenario can be partially probed at the LHC.Comment: Version with Erratum. Numerical bug fixed. An upper bound on the
reheating temperature strengthened by about an order of magnitud
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